ORIGINAL PAPER Influence of varying nitrogen levels on lipid accumulation in Chlorella sp. B. Kiran 1,2 • K. Pathak 1 • R. Kumar 1 • D. Deshmukh 1 • N. Rani 3 Received: 21 January 2016 / Revised: 6 May 2016 / Accepted: 18 May 2016 / Published online: 2 June 2016 Ó Islamic Azad University (IAU) 2016 Abstract Microalgal lipids can be enhanced through varying nitrogen (N) content, and limited supply of nitro- gen source seems to be valuable approach for increased lipid accumulation in microalgae. In this study, Chlorella sp. IM-02 was observed under fluorescence microscope for increased number of lipid bodies under nitrogen scarcity. Fourier transform infrared spectroscopy was used to determine spectral changes due to varying lipid content under nitrogen-starved (N 0 , without sodium nitrate), nitrogen-limited (N 0.1 ,N 0.25 ,N 0.5 and N 1.0 representing 0.1, 0.25, 0.5 and 1.0 g/L of sodium nitrate, respectively) and nitrogen-sufficient (N 1.5 , i.e., 1.5 g/L sodium nitrate) setting. Chlorophyll content was also monitored under these conditions as growth indicator. Various biochemical components viz. total carbohydrates, total proteins and total lipids were also estimated under varying nitrogen levels spectrophotometrically. On fourth day itself, maxi- mum lipid productivity was observed in case of N 0.5 , which is having one-third of nitrogen concentration present in original growth media, BG-11. This concludes N 0.5 as suitable nitrogen provision for better production of lipids in Chlorella sp. IM-02 without much compromising the bio- mass production as both growth and lipid quantity are key parameters affecting the lipid productivity of any microalgal strain. Keywords Microalgae Á Nutrient stress Á Wastewater Á Fluorescence microscopy Introduction Biofuels are usually considered as the most promising solution to world energy crisis due to their biodegradabil- ity, sustainability and environmental benign nature. Microalgae are emerging as most sustainable candidate for biofuel production. Some advantages such as easily cul- tivable, usage of non-arable land, no competition with food crops for land and water, nutrients uptake from wastewater, ability to produce substantial amount of triacylglycerides (TAG) and shorter doubling time make them better than other available feedstocks (Kiran et al. 2014a; Sharma et al. 2012; Wang et al. 2009). Microalgae represent rare group of microorganisms that can easily adapt to almost all ecological habitats, with autotrophic, heterotrophic or mixotrophic behavior (John et al. 2011). Autotrophic microalgae can fix carbon dioxide present in the atmosphere in the presence of sunlight through the process of photosynthesis, whereas hetero- trophs can utilize organic carbon present in environment for the synthesis of their building blocks. Mixotrophic microalgae can either use inorganic or use organic carbon depending upon availability and requirement. Microalgal lipid content is dependent on various envi- ronmental parameters viz. temperature (Zhu et al. 2009), concentration of nutrients (Gouveia and Oliveira 2009) and light intensity (Yeesang and Cheirsilp 2011). The average lipid content in microalgae is up to 50 % of dry weight, but under stress conditions some species are reported to achieve lipid content up to 70 % (Ma 2006; Xin et al. 2010). Total algal lipids are comprised of both polar (such & B. Kiran kiranb@iiti.ac.in 1 Department of Mechanical Engineering, Indian Institute of Technology, Indore 452020, India 2 Centre for Biosciences and Biomedical Engineering, Indian Institute of Technology, Indore 452020, India 3 Department of Bioscience, Himachal Pradesh University, Summer Hill, Shimla, HP 171005, India 123 Int. J. Environ. Sci. Technol. (2016) 13:1823–1832 DOI 10.1007/s13762-016-1021-4